This study will evaluate the impact of exposure to xenobiotic chemicals, such as polychlorinated biphenyls (PCBs) and polybrominated biphenyls (PBBs) on physical, cognitive, and neurological development in early childhood. Two cohorts identified by the investigators in connection with previous research will be studied: (a) children exposed to moderate levels of PCBs, whose developmental status has been assessed at birth and later in infancy; and (b) children exposed to high levels of PCBs and/or PBBs, whose mothers provided serum samples at or about the time of their birth. All children will be assessed at age 4 years. A 10 cc serum sample will be obtained from each child and analyzed using packed column gas chromatography for levels of PCBs, PBBs, and nine other environmental contaminants. Each child will be evaluated with a comprehensive psychological test battery and pediatric examinations. Mothers will provide background information including socioeconomic status, family size, pregnancy and delivery complications, child's health history, and stress experienced by the family. The Home Observation for Measurement of the Environment and a maternal IQ test will also be administered. Because evaluation of human exposure to chemical contaminants is by necessity correlational, a broad range of potential confounding variables will be assessed in relation to each measure of toxic exposure. Those control variables that are correlated at an alpha level less than or equal to .10 with a given measure of exposure will be controlled statistically in all analyses based on that measure. Since the children are expected to exhibit a range of exposure levels, multiple regression analysis will be the primary statistical technique. Dose-response relationships will be examined using analysis of covariance. Cord serum and maternal breast milk PCB and PBB levels, which are available for approximately half the sample, will be used to assess the relative contribution of intrauterine and postnatal nursing exposure to developmental deficits observed at 4 years. A 10% sample of the serum specimens will be reanalyzed using capillary column gas chromatography to identify specific toxic PCB and PBB congeners associated with observed deficits.